This invention relates generally to television signal tuning systems and particularly to cable antenna television (CATV) tuning systems. In so-called pole mounted cable television systems, each individual cable subscriber has a simple cable input to his terminal (television receiver). In these systems, the full functional control afforded by a television receiver (or video cassette recorder) is retained. A difficulty arises with respect to premium channels (usually scrambled) for which an extra charge is imposed by the cable head-end.
Many types of interference systems are in use in which the premium channel signals are scrambled by an interfering signal generated at the pole installation. The subscriber cannot receive an intelligible picture even though his receiver is tuned to the appropriate channel frequency. Such systems, while effective, require traps to be set at the pole sites and are costly because they are labor intensive.
The system of the invention uses a novel local oscillator arrangement for achieving an economical cable decryption system. For maximum benefit, the invention discloses taking advantage of the placement of encrypted channels within particular groups of cable channels. For example, conventional, non-premium, NTSC television channels are allocated to a given frequency band and are provided in unscrambled form directly to individual cable subscribers as part of their basic service. Premium or scrambled channels are included in groups of frequencies in different bands and are available to individual subscribers only upon payment of an appropriate fee. The premium channels are unscrambled by a local oscillator that converts the scrambled signal to an intermediate frequency signal. Decryption is performed and the oscillator remodulates the intermediate frequency signal back to the frequency of the incoming channel, which is provided, along with the bypassed unscrambled channels, to the subscriber.
A feature of this invention is the provision of a local oscillator in a channel tuning module that covers a limited group of contiguous channels (six in the preferred embodiment). The local oscillator is tunable by the cable head-end to select any one of the six television channel signals in the group for conversion to a fixed intermediate frequency signal. Descrambling is performed either at intermediate frequency or at baseband frequency to develop a processed (unscrambled) video signal which is remodulated by the same local oscillator to the RF channel frequency of the original incoming television signal. The remodulated signal (at the same channel frequency) is supplied to the subscriber along with the bypassed unscrambled television signals. The modular approach and the provision of a single local oscillator for both demodulation and remodulation produces a very attractive cable converter system. The system eliminates the need for separate remote control devices since the subscriber uses the existing television receiver tuner, and eliminates the interaction between the user and the cable decoder, which makes the system much simpler.